It would look presumptive to start naming his cabinet before the election and would put him under pressure to guarantee others their jobs.

Ed Miliband and Ed Balls at the Labour conference in Brighton last year. Photograph: Getty Images.

In his interview on The Andrew Marr Show this morning, Ed Miliband again guaranteed that Ed Balls would be shadow chancellor at the time of the general election. Asked "is Ed Balls safe in his job at the moment?" he replied: "Ed Balls is doing a really good job and, absolutely, I've said that he's going to be the shadow chancellor going with me into the election". That should put an end to speculation that Alistair Darling could return to his old job after the Scottish independence referendum, or that Chuka Umunna or Rachel Reeves could be rapidly promoted. But someasked why Miliband didn't go further and pledge that Balls would serve as chancellor in a Labour government. Is he planning to replace him after May 2015? Is he holding out the post for the Lib Dems? The speculation goes on.

But as one Labour source pointed out to me this morning, there are two considerations that likely explain why Miliband chose not to give this guarantee. The first is that it would look "presumptive" for him to start announcing what jobs his shadow cabinet ministers will do in government (akin to "measuring up the curtains"). It would give the impression that Labour believes the voters have already made up their minds. The second is that guaranteeing Balls will serve as chancellor would inevitably lead to speculation about other top positions. Will Harriet Harman be deputy prime minister? Will Douglas Alexander be foreign secretary? Miliband can't have one rule for Balls and another for them. For these reasons, much as journalists may wish otherwise, don't expect Miliband to start naming his cabinet in advance of 7 May 2015.

Miliband went on to say of Balls: "People have their critics; the thing I'd say to you about Ed Balls? He's got a clear sense of what this economy needs, he's working with me on tackling the cost-of-living crisis that we face and he's got the toughness to stand up to lots of people who want more spending when actually it's going to be tough for Labour."